Amino-acid code

Nucleobase A
Nucleobase B
Nucleobase C

Crick, Francis H.C./ Barnett, Leslie/ Brenner, S./ Watts-Tobin, R.J. (1961). General Nature Of The Genetic Code For Proteins. Nature. 192 (4809): 1227–32. Nature Publishing Group. S.1227.

There is now a mass of indirect evidence which suggests that the amino-acid sequence along the polypeptide chain of a protein is determined by the sequence of the bases along some particular part of the nucleic acid of the genetic material. Since there are twenty common amino-acids found throughout Nature, but only four common bases, it has often been surmised that the sequence of the four bases is in some way a code for the sequence of the amino-acids. In this article we report genetic experiments which, together with the work of others, suggest that the genetic code is of the following general type:

(a) A group of three bases (or, less likely, a multiple of three bases) codes one amino-acid.
(b) The code is not of the overlapping type (see Fig. 1).
(c) The sequence of the bases is read from a fixed starting point. This determines how the long sequences of bases are to be correctly read off as triplets. There are no special ‚commas to show how to select the right triplets. If the starting point is displaced by one base, then the reading into triplets is displaced, and thus becomes incorrect.
(d) The code is probably ‚degenerate‘; that is, in general, one particular amino-acid can be coded by one of several triplets of bases.